This combines two updates into one: It creates a new manager, that
takes care of everything related to handling the submission directory
itself.  And it defines a series of modes for creating the submission
directory.

The modes for the submission directory are probably more interesting
for the users.  There are now for modes:

* "no_clobber": create the directory and if that fails raise an error.
  this is the default behavior we had so far.

* "overwrite": create the directory, removing anything in the way.
  this was the behavior so far when `optRemoveSubmitDir` was
  specified.

* "unique": append a unique suffix to the submit directory name.  this
  is mostly meant for unit-tests in which one may specify the same
  submit directory multiple times or in multiple tests.

* "unique-link": same as "unique" but create a link under the
  officially requested name to point to the newly created directory,
  replacing any previously existing link.  the idea is that this is
  mostly consistent with the old behavior while not requiring the user
  to remove the submit directory, and indeed keeping a history of all
  past submissions.

The unique directories will be named something like
`submitDir-2019-08-22-1438-95f6`, which is essentially the originally
requested name, the date-time, and some random digits to make this
unique if several submissions happen in quick succession.  The date
comes first so that directories are ordered by date-time.  The actual
date format can be configured, if desired.

I did not change the default behavior (hopefully), but if people like
it we could probably make "unique-link" the default mode instead of
"no-clobber".